This invention relates to the field of visual display (CRT) devices and more particularly to an improved circuit arrangement for spacing the data rows on a display screen.
There are many applications for alphanumeric displays, including airline terminal monitors and computer data displays. A typical format might be a five-by-seven array of dots to form the characters, utilizing seven horizontal lines (no interlace) with three lines for spacing between rows of characters. Added clarity can be gained by simply using more horizontal lines for spacing, but this obviously reduces the maximum number of rows of data that can be displayed concurrently if the standard horizontal line rate is used. Likewise, more data could be displayed with reduced clarity by using fewer lines for spacing. One compromise solution is to use interlaced scan as in broadcast television, but the resulting 30 Hz flicker caused by the displaced fields requires the use of phosphors with longer persistence. Such phosphors, however, have poorer spot definition and are subject to "burn" or permanent darkening of the phosphor. Interlaced scan may also require that lines of characters begin on odd and even lines alternately in order to minimize field-to-field variation. Another approach is to lower the vertical scan rate from 60 Hz to 50 Hz, but this requires great care in shielding 60 Hz interference from the CRT device. Increasing the number of horizontal lines is still another possibility but, because both dot and character rates are increased proportionately, this solution is limited by the "roll-off" of the cable. A better solution is to "step" the vertical scan by providing a pulse to it during one of the horizontal lines between each two rows of characters.
The latter has been accomplished in the prior art by running a separate line to the display device and sending a stepping pulse when extra spacing was desired since the extra pulse cannot easily be combined with and separated from the video signal. As the display device may be a monitor in an airport or other remote location, it would be advantageous to eliminate the need for this extra cable.